1. Field of the Invention
This invention pertains generally to circuit interrupters and, more particularly, to circuit breakers including an electronic trip circuit and a trip actuator.
2. Background Information
Circuit interrupters include, for example, circuit breakers, contactors, motor starters, motor controllers, other load controllers and receptacles having a trip mechanism. Circuit breakers are generally old and well known in the art. Examples of circuit breakers are disclosed in U.S. Pat. Nos. 5,260,676; and 5,293,522.
Circuit breakers are used to protect electrical circuitry from damage due to an overcurrent condition, such as an overload condition or a relatively high level short circuit or fault condition. In small circuit breakers, commonly referred to as miniature circuit breakers, used for residential and light commercial applications, such protection is typically provided by a thermal-magnetic trip device. This trip device includes a bimetal which is heated and bends in response to a persistent overcurrent condition. The bimetal, in turn, unlatches a spring powered operating mechanism which opens the separable contacts of the circuit breaker to interrupt current flow in the protected power system. An armature, which is attracted by the sizable magnetic forces generated by a short circuit or fault, also unlatches, or trips, the operating mechanism.
Miniature circuit breakers use bimetals or analog circuits to provide overload (thermal) protection. Bimetals do a good job of simulating thermal cooling of power conductors. The bimetal trips a circuit breaker when its temperature reaches a certain predetermined value. Most of today's circuit breakers are not ambient temperature compensated.
UL 489 is a molded case circuit breaker standard that controls tripping characteristics. For a circuit breaker rated at, for example, 30 A or less, the following performance is required at three different current levels relative to the rated current: (1) 200%: tripping in greater than 12 seconds but less than 2 minutes; (2) 135%: tripping in less than 1 hour; and (3) 100%: no tripping.
Analog circuits can simulate cooling using charge stored on a capacitor, which is simply reset to a fixed thermal level after a trip. See, for example, U.S. Pat. No. 5,418,677.
Some analog circuits may use the temperature of an internal shunt for tripping, but this technique suffers from ambient temperature calibration issues or inaccuracies at the, above, 135% must trip setting of UL 489.
Accordingly, there is room for improvement in circuit interrupters.